1. Field of the Invention
This invention relates to a GaN single-crystal substrate and a method for producing a GaN single crystal. Particularly, the invention relates to a GaN single-crystal substrate and a method for producing a GaN single crystal, where abnormal growth can be suppressed.
2. Description of the Related Art
GaN-based compound semiconductors, such as group III nitride compound semiconductors of gallium nitrides (GaN), indium gallium nitrides (InGaN), aluminum gallium nitrides (AlGaN), etc. are used as materials for light-emitting devices, such as light-emitting diodes (LEDs), laser diodes (LDs), etc. GaN-based compound semiconductors are also used in elements for electronic devices because of their excellent heat- and environment-resistances.
As GaN free-standing substrate production methods used in such light-emitting devices or elements for electronic devices, there are developed ultrahigh-temperature high-pressure methods, flux methods, halide vapor phase epitaxy (HVPE) methods, etc., for example. Here, although GaN free-standing substrates are obtained by HVPE, epitaxially growing a specified compound semiconductor layer on a GaN free-standing substrate formed by conventional HVPE may cause the problem of abnormal growth causing partial pits and protrusions in substrate surface after epitaxial growth (See, e.g., Liliental-Weber et. al., Journal of Electronic Materials, May 2005, p. 605).
The GaN free-standing substrate obtained by the method disclosed by Liliental-Weber et. al. has an abnormally grown region in its surface. To epitaxially grow a light-emitting device structure on such a GaN free-standing substrate causes disturbance in the semiconductor layer epitaxially grown on the abnormally grown region. Accordingly, the GaN free-standing substrate obtained by the method disclosed by Liliental-Weber et al. causes difficulty in enhancing light-emitting device yields.